High-Cr seamless steel pipes or tubes containing, by mass %, 10.50 to 14.00% of Cr have been increasingly used for oil and gas wells, for various plants and for building structures.
By the way, a piercing and rolling method using an inclined roll type piercing mill (hereinafter often referred to as “piercing mill”) is frequently applied to the recent piercing and rolling for high Cr-seamless steel pipes or tubes. Concretely, a hollow pipe or tube stock is manufactured from a solid steel stock with a round section (hereinafter referred to as “round billet” or simply as “billet”) by use of a piercing mill. Then, the pipe or tube stock is rolled by an elongator such as a mandrel mill, a plug mill, an Assel mill, or a push bench to reduce the wall thickness thereof, and the outer diameter thereof is then narrowed by use of a constant diameter mill such as a stretch reducing mill or a sizing mill.
However, since the hot workability of high-Cr steel is lower than that of a so-called “common steel”, the piercing and rolling by a piercing mill thereof tends to cause defects on the internal surface of the resulting pipe or tube stock. Typical examples of the defects on the internal surface of the pipe or tube stock include a so-called “internal fracture flaw”, which may be called often “internal scab” or “lapping mark”.
The internal surface defects of steel pipes or tubes may be roughly distinguished between defects resulting from internal surface defects and/or hot workability of the billet itself, that is to say, “material-originated flaw”, and defects resulting from surface defects of an internal surface tool such as a piercing plug or bar used for pipe or tube making and/or pipe or tube making conditions or the like, that is to say, “machine-originated flaw”. According to this view of classification, it is realized that the former “material-originated flaw” occurs mainly in the piercing and rolling stage, and the major flaw thereof is an internal fracture flaw.
The elimination of internal surface defects on steel pipes or tubes needs a lot of man-hours for treatments of resulting steel pipes or tubes in a different process, for removals of flawed portions from resulting steel pipes or tubes by cutting, or the like. Consequently, it leads to a remarkable reduction in production efficiency. Further, a deep flaw leads to disposal of the steel pipe or tube itself, resulting in deterioration of yield.
Therefore, the following techniques in the Patent Documents 1 to 6 are proposed for suppress internal surface defects on pipe or tube stock in the piercing and rolling stage which are directly linked to the internal surface defects on resulting steel pipes or tubes.
The Patent Document 1 discloses a technique for enhancing the hot workability of a steel stock in the piercing and rolling by a piercing mill while minimizing the contents of P and S that are impurity elements in a steel, thereby suppressing internal fracture flaws.
The Patent Document 2 discloses a technique for suppressing the production of δ-ferrite by reducing the heating temperature of a billet, that is the steel stock, and also by suppressing the temperature rise involved by work heat generation through reducing the average strain rate in the piercing and rolling by a piercing mill, thereby preventing the occurrence of internal fracture flaws.
The Patent Document 3 discloses a method for manufacturing a martensitic seamless steel pipe or tube, capable of improving the microstructure in hot working by regulating the contents of the specified alloy components, controlling the annealing heating time, and further setting the piercing temperature lower than 1200° C.
The Patent Document 4 discloses a technique for performing piercing and rolling while adjusting, in the piercing and rolling by a piercing mill with disk roll-type guide shoes, the diameter of a steel stock, the distance from the starting position of gripping the steel stock with inclined rolls to the tip of a plug, the clearance between guide shoes at the position of the tip of a plug, the inclined roll gap and the clearance between the guide shoes at the wall thickness determining position.
The Patent Document 5 discloses a method for manufacturing a high-Cr seamless steel pipe or tube, capable of improving the microstructure in hot working by regulating the content of Cr, the contents of S and P as impurity elements, and the contents of elements to be added to the high-Cr steel, and then adjusting the soaking time of the casting bloom or the billet, the soaking time of the rolling stock, and the heating time in pipe or tube making, thereby preventing internal surface defects.
The Patent Document 6 discloses a method for manufacturing a martensitic stainless seamless steel pipe or tube, capable of improving the microstructure in hot working by regulating the contents of specified alloy components and adjusting the cross angle and the feed angle at the time of piercing and rolling, thereby preventing internal surface defects.
Patent Document 1: Japanese Laid-Open Patent Publication No. 59-208055,
Patent Document 2: Japanese Laid-Open Patent Publication No. 63-281705,
Patent Document 3: Japanese Laid-Open Patent Publication No. 4-224659,
Patent Document 4: Japanese Laid-Open Patent Publication No. 5-69011,
Patent Document 5: Japanese Laid-Open Patent Publication No. 2003-3212,
Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-43935.